EP0580452A1 - Field effect trench transistor having lightly doped epitaxial region on the surface portion thereof - Google Patents
Field effect trench transistor having lightly doped epitaxial region on the surface portion thereof Download PDFInfo
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- EP0580452A1 EP0580452A1 EP19930305856 EP93305856A EP0580452A1 EP 0580452 A1 EP0580452 A1 EP 0580452A1 EP 19930305856 EP19930305856 EP 19930305856 EP 93305856 A EP93305856 A EP 93305856A EP 0580452 A1 EP0580452 A1 EP 0580452A1
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- trench
- epitaxial layer
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- 230000005669 field effect Effects 0.000 title claims abstract description 10
- 239000000758 substrate Substances 0.000 claims description 28
- 210000000746 body region Anatomy 0.000 claims description 21
- 238000000034 method Methods 0.000 claims description 6
- 238000004519 manufacturing process Methods 0.000 claims description 2
- 239000007772 electrode material Substances 0.000 claims 1
- 230000015556 catabolic process Effects 0.000 abstract description 9
- 230000005684 electric field Effects 0.000 abstract description 6
- 230000002411 adverse Effects 0.000 abstract description 2
- 230000000694 effects Effects 0.000 abstract 1
- 229910052785 arsenic Inorganic materials 0.000 description 7
- RQNWIZPPADIBDY-UHFFFAOYSA-N arsenic atom Chemical compound [As] RQNWIZPPADIBDY-UHFFFAOYSA-N 0.000 description 7
- BHEPBYXIRTUNPN-UHFFFAOYSA-N hydridophosphorus(.) (triplet) Chemical compound [PH] BHEPBYXIRTUNPN-UHFFFAOYSA-N 0.000 description 5
- 238000002513 implantation Methods 0.000 description 5
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 description 4
- 229910021420 polycrystalline silicon Inorganic materials 0.000 description 4
- ZOXJGFHDIHLPTG-UHFFFAOYSA-N Boron Chemical compound [B] ZOXJGFHDIHLPTG-UHFFFAOYSA-N 0.000 description 3
- 229910052796 boron Inorganic materials 0.000 description 3
- 229920005591 polysilicon Polymers 0.000 description 3
- 238000000637 aluminium metallisation Methods 0.000 description 2
- 238000009792 diffusion process Methods 0.000 description 2
- 230000000873 masking effect Effects 0.000 description 2
- 239000000463 material Substances 0.000 description 2
- FAIAAWCVCHQXDN-UHFFFAOYSA-N phosphorus trichloride Chemical compound ClP(Cl)Cl FAIAAWCVCHQXDN-UHFFFAOYSA-N 0.000 description 2
- 230000008569 process Effects 0.000 description 2
- 239000004065 semiconductor Substances 0.000 description 2
- 235000012239 silicon dioxide Nutrition 0.000 description 2
- 239000000377 silicon dioxide Substances 0.000 description 2
- JBRZTFJDHDCESZ-UHFFFAOYSA-N AsGa Chemical compound [As]#[Ga] JBRZTFJDHDCESZ-UHFFFAOYSA-N 0.000 description 1
- 229910052582 BN Inorganic materials 0.000 description 1
- PZNSFCLAULLKQX-UHFFFAOYSA-N Boron nitride Chemical compound N#B PZNSFCLAULLKQX-UHFFFAOYSA-N 0.000 description 1
- 229910001218 Gallium arsenide Inorganic materials 0.000 description 1
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 description 1
- 230000015572 biosynthetic process Effects 0.000 description 1
- 239000005380 borophosphosilicate glass Substances 0.000 description 1
- 230000001419 dependent effect Effects 0.000 description 1
- 239000002019 doping agent Substances 0.000 description 1
- 230000002349 favourable effect Effects 0.000 description 1
- 239000007943 implant Substances 0.000 description 1
- 238000001465 metallisation Methods 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 230000002028 premature Effects 0.000 description 1
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- 229910052710 silicon Inorganic materials 0.000 description 1
- 239000010703 silicon Substances 0.000 description 1
- 230000007480 spreading Effects 0.000 description 1
- 230000007704 transition Effects 0.000 description 1
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L29/00—Semiconductor devices adapted for rectifying, amplifying, oscillating or switching, or capacitors or resistors with at least one potential-jump barrier or surface barrier, e.g. PN junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
- H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
- H01L29/68—Types of semiconductor device ; Multistep manufacturing processes therefor controllable by only the electric current supplied, or only the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched
- H01L29/76—Unipolar devices, e.g. field effect transistors
- H01L29/772—Field effect transistors
- H01L29/78—Field effect transistors with field effect produced by an insulated gate
- H01L29/7801—DMOS transistors, i.e. MISFETs with a channel accommodating body or base region adjoining a drain drift region
- H01L29/7802—Vertical DMOS transistors, i.e. VDMOS transistors
- H01L29/7813—Vertical DMOS transistors, i.e. VDMOS transistors with trench gate electrode, e.g. UMOS transistors
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L29/00—Semiconductor devices adapted for rectifying, amplifying, oscillating or switching, or capacitors or resistors with at least one potential-jump barrier or surface barrier, e.g. PN junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
- H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
- H01L29/68—Types of semiconductor device ; Multistep manufacturing processes therefor controllable by only the electric current supplied, or only the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched
- H01L29/76—Unipolar devices, e.g. field effect transistors
- H01L29/772—Field effect transistors
- H01L29/78—Field effect transistors with field effect produced by an insulated gate
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L29/00—Semiconductor devices adapted for rectifying, amplifying, oscillating or switching, or capacitors or resistors with at least one potential-jump barrier or surface barrier, e.g. PN junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
- H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
- H01L29/06—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by their shape; characterised by the shapes, relative sizes, or dispositions of the semiconductor regions ; characterised by the concentration or distribution of impurities within semiconductor regions
- H01L29/08—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by their shape; characterised by the shapes, relative sizes, or dispositions of the semiconductor regions ; characterised by the concentration or distribution of impurities within semiconductor regions with semiconductor regions connected to an electrode carrying current to be rectified, amplified or switched and such electrode being part of a semiconductor device which comprises three or more electrodes
- H01L29/0843—Source or drain regions of field-effect devices
- H01L29/0847—Source or drain regions of field-effect devices of field-effect transistors with insulated gate
- H01L29/0852—Source or drain regions of field-effect devices of field-effect transistors with insulated gate of DMOS transistors
- H01L29/0873—Drain regions
- H01L29/0878—Impurity concentration or distribution
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L29/00—Semiconductor devices adapted for rectifying, amplifying, oscillating or switching, or capacitors or resistors with at least one potential-jump barrier or surface barrier, e.g. PN junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
- H01L29/40—Electrodes ; Multistep manufacturing processes therefor
- H01L29/41—Electrodes ; Multistep manufacturing processes therefor characterised by their shape, relative sizes or dispositions
- H01L29/423—Electrodes ; Multistep manufacturing processes therefor characterised by their shape, relative sizes or dispositions not carrying the current to be rectified, amplified or switched
- H01L29/42312—Gate electrodes for field effect devices
- H01L29/42316—Gate electrodes for field effect devices for field-effect transistors
- H01L29/4232—Gate electrodes for field effect devices for field-effect transistors with insulated gate
- H01L29/42356—Disposition, e.g. buried gate electrode
- H01L29/4236—Disposition, e.g. buried gate electrode within a trench, e.g. trench gate electrode, groove gate electrode
Definitions
- This invention is directed to field effect transistors, and especially to field effect transistors for power applications and in which the transistor gate is located in a trench in a substrate having a lightly doped epitaxial layer at the principal surface of the substrate.
- DMOS transistor double diffused MOS transistors
- a typical such transistor shown in Fig. 1 includes a substrate 10 doped N+, an epitaxial layer 14 formed thereon doped N-, a body region 16 doped P, and a source region 20 doped N+.
- the gate electrode 22 is typically conductive polysilicon formed in a trench 24 which may be V-shaped, U-shaped, or a truncated V as shown in Fig. 1.
- the source contacts 26 short the body region 16 to the source region 20, and the drain contact 30 is formed on the substrate 10 backside.
- the channel length designated by X is the length of the P body region 16 adjacent to the gate electrode 22. It is to be understood that the structure of Fig. 1 is illustrative; in other devices which are also well known, the trench 24 is completely filled with the gate electrode 22, thus establishing a planar principal surface.
- the channel region of such a device is the P body diffusion. To achieve low channel resistance, this region is kept short. It is important that the trench extends slightly beyond the depth of the P body region.
- a lightly doped second epitaxial region is formed on the top portion of the usual epitaxial layer.
- the upper epitaxial layer extends slightly deeper than the bottom of the trench, and the upper epitaxial layer is less doped than is the underlying epitaxial layer.
- the lightly doped upper epitaxial layer reduces the electric field around the bottom of the trench, to protect the gate oxide in the trench from breakdown during high voltage operation.
- the transistor As the transistor is turned on, current flows down the surface of the trench through the channel, i.e. along the side of the trench adjacent to the P body region, to the lower portion of the trench surface, and spreading from the bottom surface of the trench down to the drift region (the second epitaxial layer) and to the drain region (the substrate).
- the upper portion of the lightly doped top epitaxial layer has little adverse impact on the source-drain on resistance.
- the portion of the upper epitaxial layer that extends below the bottom of the trench does undesirably contribute some extra on resistance.
- this portion of the upper epitaxial layer is very thin, the added on resistance is small.
- the structure in accordance with the invention can be achieved through a modified epitaxial growth process without significant extra cost or additional masking steps, and is efficiently controlled using conventional semiconductor processing equipment.
- Fig. 1 shows a prior art DMOS trenched field effect transistor.
- Fig. 2 shows a transistor in accordance with the present invention.
- Figs. 3a through 3g show process steps to form the transistor of Fig. 2.
- Fig. 2 shows a single transistor in accordance with the present invention. It is to be understood that as is shown at the left and right hand portions of Fig. 2, the structure is replicated in the typical cell-like power transistor structure to provide many such transistors connected in parallel, typically for power switching applications. The chief application of DMOS transistors is for power switching; however, the present invention is not limited thereto.
- the transistor of Fig. 2 includes conventional heavily doped N+ substrate 40 and an N doped epitaxial layer 42, the doping concentration of which is strongly dependent on the voltage application.
- the doping level (phosphorous or arsenic) of N+ substrate 40 is 6x1018 to 1x1020 ion/cm2.
- the doping level (arsenic or phosphorous) of the lower N epitaxial layer 42 is such as to achieve a sheet resistance of approximately 0.5 to 1.0 ohms ⁇ cm. (It is to be understood that the polarities herein would be reversed for an N-channel device, as against the present P-channel device.)
- the thickness of the lower epitaxial layer 42 is approximately 3 to 8 microns (micrometers), depending on different applications.
- a more lightly doped second (upper) epitaxial layer 46 which is arsenic doped N such as to achieve a sheet resistance of approximately 1.0 to 2.0 ohms ⁇ cm.
- the resistivity of the upper epitaxial layer 46 is typically twice that of the lower epitaxial layer 42; hence the doping level of the upper layer is about half that of the lower layer.
- the thickness of the upper epitaxial layer 46 is approximately 2 to 3 microns.
- the P doped body region 50 includes a shallow P channel region 51 and a deep P+ region 53.
- the shallow P channel region 51 is doped to a level of approximately 2x1013 to 1x1014 ion/cm2, while the deep P+ region 53 is doped to 5x1014 to 1x1016 ion/cm2.
- an N+ source region 52 doped to a level of approximately 6x1014 to 1x1016 ion/cm2 using again arsenic or phosphorous.
- the depth of the N+ source region 52 is approximately 0.3 to 0.5 microns.
- the P body region 50 extends down approximately 0.5 to 1.5 microns into the lower epitaxial layer 42 (as shown).
- the U-shaped trench 54 is conventionally lined with gate oxide 56 and then filled with doped polysilicon 60.
- the sheet resistance of the doped polysilicon 60 is approximately 20 to 40 ohm/square using phosphorous chloride as the dopant.
- insulating gate oxide layer 64 Conventionally formed over the principal surface of the device is insulating gate oxide layer 64 and overlying that is the conventional aluminum metallization layer 68 which contacts the source 52 and body regions 50. It is to be understood that the additional gate structures and source regions 70, 72 and 78, 80 shown respectively in the left hand and right hand portions of Fig. 2 are portions of adjacent identical transistor cells.
- Substrate 40 serves as a drain region and is conventionally contacted by metallization formed on the backside of substrate 40 (not shown).
- the P body region 50 extends lower than the depth of the trench 54. However, it does not so extend at the portions immediately adjacent to the bottom portion of the trench 54, but only at locations spaced away from the bottom portion of the trench 54.
- the trench is conventionally U-shaped, i.e. having approximately vertical walls.
- the present invention is also applicable to transistors having a V-shaped or a truncated V-shaped trench.
- Fig. 2 intentionally reduces the doping transition at the lower corners of the trench 54, thus locally reducing the strength of the electric fields. This is achieved by providing the lightly doped upper epitaxial region 46 adjacent to the bottom portions of the trench 54.
- trench 54 is approximately 1.5 microns deep, leaving a distance "d" of approximately 0.5 microns between the bottom of trench 54 and the upper surface of lower epitaxial layer 42. This distance d is established to avoid early breakdown even at the trench corners. Since the deep P+ body region extends down into the lower epitaxial layer 42, this results in P-N junction breakdown to eliminate walk-out phenomena.
- the structure shown in Fig. 2 is illustrative for a transistor for applications of up to 60 volts.
- FIG. 3a Fabrication of the structure of Fig. 2 is illustrated in Figures 3a to 3g.
- Fig. 3a one begins conventionally with an N+ doped substrate 40. Then one conventionally grows a first epitaxial layer 42 doped N type to a level of about 0.5 to 1.2 ohms ⁇ cm as shown in Fig. 3b. Then as shown in Fig. 3c a second epitaxial layer 46 is grown on the first epitaxial layer 42, the second epitaxial layer 46 being more lightly doped to a level of approximately 1.0 to 2.0 ohms ⁇ cm. All three regions 40, 42, 46 are doped using arsenic or phosphorous with arsenic preferred.
- the two epitaxial layers 42, 46 have a thickness of approximately 3 to 8 microns ⁇ m, and 2 to 3 ⁇ m, respectively.
- the shallow P body in region 50 is formed by boron implantation at 60 keV using a dose of 2x1013 to 1x1014 ion/cm2. This implantation is then driven into a depth of approximately 1.0 to 2.0 microns.
- the deep P body in region 50 is then implanted in the central portion of the transistor using an implant dose of boron of 5x1014 to 1x1016 ion/cm2 at an energy of 60 KeV. This implantation is then driven in to a depth of approximately 2.0 to 3.5 microns.
- the deep P body in region 50 can be formed with a boron nitride process.
- the resulting sheet resistance is about 20 to 100 ohm/square.
- the N+ region 52 is conventionally implanted using a mask.
- the implantation is at an energy of 80 KeV and a dosage of 6x1015 to 1x1016 ion/cm2, again using arsenic or phosphorous.
- the N+ region 52 is then driven in (diffused) to a depth of approximately 0.3 to 0.5 microns. (Also shown are N+ source regions 72, 80 of adjacent transistors which are formed simultaneously).
- a U-shaped trench 54 is conventionally reactive ion etched through the central portion of the source region 52 into the body region 50 to a depth of approximately 1.0 to 2.0 microns.
- the trench 54 is then conventionally lined (see Fig. 3g) with silicon dioxide 56 by thermal growth of silicon dioxide on the trench walls to a thickness of approximately 500 to 1000 ⁇ . Then the lined trench 54 is conventionally filled with polycrystalline silicon 60 which is then doped with phosphorous chloride to a sheet resistance of approximately 20 to 40 ohms/square. Then (not shown) an insulating (e.g. BPSG) layer is conventionally deposited and patterned for insulating the gate electrode. Then (not shown) the aluminum metallization layer is conventionally formed thereover, establishing electrical contact to the body and source regions.
- insulating e.g. BPSG
- the final configuration of the device as shown in Fig. 2 with regard to channel length is a result of the double diffusion, i.e. diffusing the N+ source region 52 after establishing the P body region 50.
- Fig. 2 may be formed by methods and materials other than as described above, so long as they result in two upper layers of the substrate, the upper most layer having a lower doping level than the layer immediately beneath it. This is most easily achieved through the above described double epitaxial layer growth, but other methods of establishing such a structure will be apparent to one of ordinary skill in the art.
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Abstract
Description
- This invention is directed to field effect transistors, and especially to field effect transistors for power applications and in which the transistor gate is located in a trench in a substrate having a lightly doped epitaxial layer at the principal surface of the substrate.
- DMOS transistor (double diffused MOS transistors) are well known as a type of field effect transistor. A typical such transistor shown in Fig. 1 includes a
substrate 10 doped N+, anepitaxial layer 14 formed thereon doped N-, abody region 16 doped P, and asource region 20 doped N+. The gate electrode 22 is typically conductive polysilicon formed in atrench 24 which may be V-shaped, U-shaped, or a truncated V as shown in Fig. 1. The source contacts 26 short thebody region 16 to thesource region 20, and thedrain contact 30 is formed on thesubstrate 10 backside. The channel length designated by X is the length of theP body region 16 adjacent to the gate electrode 22. It is to be understood that the structure of Fig. 1 is illustrative; in other devices which are also well known, thetrench 24 is completely filled with the gate electrode 22, thus establishing a planar principal surface. - It is considered critical to the performance of such a device that a narrow channel be available. The channel region of such a device is the P body diffusion. To achieve low channel resistance, this region is kept short. It is important that the trench extends slightly beyond the depth of the P body region.
- It is known that undesirably very high electric fields form at the bottom of the trench, resulting in a significant reduction in the breakdown voltage. That is to say, when such a device is operated at high voltage, the sharp corners inside the etched trench make reliable high voltage performance difficult due to local electric fields.
- That is to say, in such a device, a high electric field around the bottom corners of the trench (also called a groove) causes premature device breakdown. This breakdown can damage the gate oxide layer and even result in permanent damage to the device. It would be highly desirable to overcome this shortcoming to achieve higher breakdown voltage without excessively increasing source-drain on resistance (RDSON).
- In accordance with the invention, a lightly doped second epitaxial region is formed on the top portion of the usual epitaxial layer. The upper epitaxial layer extends slightly deeper than the bottom of the trench, and the upper epitaxial layer is less doped than is the underlying epitaxial layer. Thus the lightly doped upper epitaxial layer reduces the electric field around the bottom of the trench, to protect the gate oxide in the trench from breakdown during high voltage operation.
- As the transistor is turned on, current flows down the surface of the trench through the channel, i.e. along the side of the trench adjacent to the P body region, to the lower portion of the trench surface, and spreading from the bottom surface of the trench down to the drift region (the second epitaxial layer) and to the drain region (the substrate).
- The upper portion of the lightly doped top epitaxial layer has little adverse impact on the source-drain on resistance. The portion of the upper epitaxial layer that extends below the bottom of the trench does undesirably contribute some extra on resistance. However, since this portion of the upper epitaxial layer is very thin, the added on resistance is small. Thus the tradeoff between increased breakdown voltage and increased on resistance is favorable. The structure in accordance with the invention can be achieved through a modified epitaxial growth process without significant extra cost or additional masking steps, and is efficiently controlled using conventional semiconductor processing equipment.
- Fig. 1 shows a prior art DMOS trenched field effect transistor.
- Fig. 2 shows a transistor in accordance with the present invention.
- Figs. 3a through 3g show process steps to form the transistor of Fig. 2.
- Fig. 2 shows a single transistor in accordance with the present invention. It is to be understood that as is shown at the left and right hand portions of Fig. 2, the structure is replicated in the typical cell-like power transistor structure to provide many such transistors connected in parallel, typically for power switching applications. The chief application of DMOS transistors is for power switching; however, the present invention is not limited thereto.
- The transistor of Fig. 2 includes conventional heavily doped
N+ substrate 40 and an N dopedepitaxial layer 42, the doping concentration of which is strongly dependent on the voltage application. The doping level (phosphorous or arsenic) ofN+ substrate 40 is 6x10¹⁸ to 1x10²⁰ ion/cm². The doping level (arsenic or phosphorous) of the lower Nepitaxial layer 42 is such as to achieve a sheet resistance of approximately 0.5 to 1.0 ohms●cm. (It is to be understood that the polarities herein would be reversed for an N-channel device, as against the present P-channel device.) The thickness of the lowerepitaxial layer 42, is approximately 3 to 8 microns (micrometers), depending on different applications. - Formed on the lower
epitaxial layer 42 is a more lightly doped second (upper)epitaxial layer 46 which is arsenic doped N such as to achieve a sheet resistance of approximately 1.0 to 2.0 ohms●cm. Thus the resistivity of the upperepitaxial layer 46 is typically twice that of the lowerepitaxial layer 42; hence the doping level of the upper layer is about half that of the lower layer. The thickness of the upperepitaxial layer 46 is approximately 2 to 3 microns. Formed in the upperepitaxial layer 46 is the P dopedbody region 50. The P dopedbody region 50 includes a shallowP channel region 51 and adeep P+ region 53. The shallowP channel region 51 is doped to a level of approximately 2x10¹³ to 1x10¹⁴ ion/cm², while thedeep P+ region 53 is doped to 5x10¹⁴ to 1x10¹⁶ ion/cm². Also formed inlayer 46 is anN+ source region 52 doped to a level of approximately 6x10¹⁴ to 1x10¹⁶ ion/cm² using again arsenic or phosphorous. The depth of theN+ source region 52 is approximately 0.3 to 0.5 microns. TheP body region 50 extends down approximately 0.5 to 1.5 microns into the lower epitaxial layer 42 (as shown). - The U-shaped
trench 54 is conventionally lined withgate oxide 56 and then filled with dopedpolysilicon 60. The sheet resistance of thedoped polysilicon 60 is approximately 20 to 40 ohm/square using phosphorous chloride as the dopant. - Conventionally formed over the principal surface of the device is insulating
gate oxide layer 64 and overlying that is the conventionalaluminum metallization layer 68 which contacts thesource 52 andbody regions 50. It is to be understood that the additional gate structures andsource regions Substrate 40 serves as a drain region and is conventionally contacted by metallization formed on the backside of substrate 40 (not shown). - Therefore, unlike what is shown in Fig. 1, in the structure of Fig. 2, the
P body region 50 extends lower than the depth of thetrench 54. However, it does not so extend at the portions immediately adjacent to the bottom portion of thetrench 54, but only at locations spaced away from the bottom portion of thetrench 54. In Fig. 2 the trench is conventionally U-shaped, i.e. having approximately vertical walls. - It is to be understood that the present invention is also applicable to transistors having a V-shaped or a truncated V-shaped trench.
- The structure of Fig. 2 intentionally reduces the doping transition at the lower corners of the
trench 54, thus locally reducing the strength of the electric fields. This is achieved by providing the lightly doped upperepitaxial region 46 adjacent to the bottom portions of thetrench 54. In one embodiment,trench 54 is approximately 1.5 microns deep, leaving a distance "d" of approximately 0.5 microns between the bottom oftrench 54 and the upper surface of lowerepitaxial layer 42. This distance d is established to avoid early breakdown even at the trench corners. Since the deep P+ body region extends down into thelower epitaxial layer 42, this results in P-N junction breakdown to eliminate walk-out phenomena. The structure shown in Fig. 2 is illustrative for a transistor for applications of up to 60 volts. - Fabrication of the structure of Fig. 2 is illustrated in Figures 3a to 3g. In Fig. 3a, one begins conventionally with an N+ doped
substrate 40. Then one conventionally grows afirst epitaxial layer 42 doped N type to a level of about 0.5 to 1.2 ohms●cm as shown in Fig. 3b. Then as shown in Fig. 3c asecond epitaxial layer 46 is grown on thefirst epitaxial layer 42, thesecond epitaxial layer 46 being more lightly doped to a level of approximately 1.0 to 2.0 ohms●cm. All threeregions epitaxial layers - Then as shown in Fig. 3d, by a conventional masking step, the shallow P body in
region 50 is formed by boron implantation at 60 keV using a dose of 2x10¹³ to 1x10¹⁴ ion/cm². This implantation is then driven into a depth of approximately 1.0 to 2.0 microns. The deep P body inregion 50 is then implanted in the central portion of the transistor using an implant dose of boron of 5x10¹⁴ to 1x10¹⁶ ion/cm² at an energy of 60 KeV. This implantation is then driven in to a depth of approximately 2.0 to 3.5 microns. - Alternatively, the deep P body in
region 50 can be formed with a boron nitride process. The resulting sheet resistance is about 20 to 100 ohm/square. - Then as shown in Fig. 3e the
N+ region 52 is conventionally implanted using a mask. The implantation is at an energy of 80 KeV and a dosage of 6x10¹⁵ to 1x10¹⁶ ion/cm², again using arsenic or phosphorous. TheN+ region 52 is then driven in (diffused) to a depth of approximately 0.3 to 0.5 microns. (Also shown areN+ source regions - Then as shown in Fig. 3f, a
U-shaped trench 54 is conventionally reactive ion etched through the central portion of thesource region 52 into thebody region 50 to a depth of approximately 1.0 to 2.0 microns. - The
trench 54 is then conventionally lined (see Fig. 3g) withsilicon dioxide 56 by thermal growth of silicon dioxide on the trench walls to a thickness of approximately 500 to 1000Å. Then the linedtrench 54 is conventionally filled withpolycrystalline silicon 60 which is then doped with phosphorous chloride to a sheet resistance of approximately 20 to 40 ohms/square. Then (not shown) an insulating (e.g. BPSG) layer is conventionally deposited and patterned for insulating the gate electrode. Then (not shown) the aluminum metallization layer is conventionally formed thereover, establishing electrical contact to the body and source regions. - It is to be understood that the final configuration of the device as shown in Fig. 2 with regard to channel length is a result of the double diffusion, i.e. diffusing the
N+ source region 52 after establishing theP body region 50. - The structure of Fig. 2 may be formed by methods and materials other than as described above, so long as they result in two upper layers of the substrate, the upper most layer having a lower doping level than the layer immediately beneath it. This is most easily achieved through the above described double epitaxial layer growth, but other methods of establishing such a structure will be apparent to one of ordinary skill in the art.
- For instance:
- (1) the P channel region may be formed after trench formation and planarization; or
- (2) the
upper epitaxial layer 46 may be formed by implantation of boron into theepitaxial layer 42; or - (3) other semiconductor materials might be used in place of silicon, for instance gallium arsenide.
- The above description is illustrative and not limiting; further modifications will be apparent to one of ordinary skill in the art in light of this specification and the appended claims.
Claims (12)
- A field effect transistor comprising:
a substrate of a first conductivity type defining a trench extending from a principal surface of the substrate;
a conductive gate electrode formed in the trench;
a source region of the first conductivity type formed in the substrate adjacent to the sidewalls of the trench and extending to the principal surface; and
a body region of a second conductivity type formed in the substrate adjacent to the sidewalls of the trench except at the lowest portion of the trench;
wherein the substrate includes a first layer of the first conductivity type and a particular doping level less than that of underlying layers of the substrate and extending from the principal surface to a depth greater than a depth of the trench, and a second layer of the first conductivity type and having a doping level greater than that of the first layer and less than that of an underlying layer of the substrate, the second layer being in a portion of the substrate deeper than the lowest portion of the trench, the layer of the substrate underlying the second layer being the drain region of the transistor. - A transistor according to claim 1, wherein the trench is U-shaped in cross-section.
- A transistor according to claim 1 or 2, wherein the first layer extends about 0.5 microns below the lowest portion of the trench.
- A transistor according to any preceding claim, wherein the first layer has a doping level about 50% that of the second layer.
- A transistor according to any preceding claim, wherein the body region extends to the principal surface only at locations spaced apart from the sidewalls of the trench.
- A transistor according to any preceding claim, wherein the first and second layers are epitaxial layers.
- A field effect transistor comprising:
a substrate of a first conductivity type being a drain region;
a first epitaxial layer of the first conductivity type formed on the substrate and having a doping level less than that of the substrate;
a second epitaxial layer of the first conductivity type formed on the first epitaxial layer and having a doping level about 50% of that of the first epitaxial layer;
a trench defined in only the second epitaxial layer and extending to within about 0.5µm of the first epitaxial layer, the trench being at least partially filled with a conductive gate electrode;
a source region of the first conductivity type formed in the second epitaxial layer and extending to a principal surface thereof adjacent to the sidewalls of the trench; and
a body region of a second conductivity type extending from a principal surface of the second epitaxial layer down to and into at least an upper portion of the first epitaxial layer and being spaced apart from the lower portion of the trench. - A method of making a field effect transistor comprising the steps of:
providing a substrate of a first conductivity type and having a principal surface;
growing a first epitaxial layer of the first conductivity type on the principal surface, the first epitaxial layer having a doping level less than that of the substrate;
growing a second epitaxial layer of the first conductivity type on the first epitaxial layer, the second epitaxial layer having a doping level less than that of the first epitaxial layer;
forming a body region of a second conductivity type in the second epitaxial layer and extending to a principal surface thereof, the body region extending at least partly into the first epitaxial layer;
forming a source region of the second conductivity type in the body region and extending to the principal surface thereof;
forming a trench extending from the principal surface of the second epitaxial layer through the source region and the body region, but not extending into the first epitaxial layer; and
filling the trench at least partially with a conductive gate electrode material. - The method of claim 8, wherein the trench extends to within 0.5 microns of the first epitaxial layer, and the level of doping of the second epitaxial layer is about 50% that of the first epitaxial layer.
- A transistor according to claims 1 to 6, wherein the first and second layers are each less then about 8µm thick.
- A field effect transistor comprising:
a substrate of a first conductivity type being a drain region;
a lower layer of the first conductivity type formed on the substrate and having a doping level less than that of the substrate;
an upper layer of the first conductivity type formed on the lower layer and having a doping level less than that of the lower layer;
a trench defined in only the upper layer and extending to within a predetermined distance of the lower layer, the trench being at least partially filled with a conductive gate electrode;
a source region of the first conductivity type formed in the upper layer and extending to a principal surface thereof adjacent to the side walls of the trench; and
a body region of a second conductivity type extending from a principal surface of the upper layer down to and into at least an upper portion of the lower layer and being spaced apart from the lower portion of the trench. - A transistor according to claim 11, wherein the upper and lower layers are epitaxial layers.
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US918954 | 1992-07-24 | ||
US07/918,954 US5910669A (en) | 1992-07-24 | 1992-07-24 | Field effect Trench transistor having lightly doped epitaxial region on the surface portion thereof |
Publications (2)
Publication Number | Publication Date |
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EP0580452A1 true EP0580452A1 (en) | 1994-01-26 |
EP0580452B1 EP0580452B1 (en) | 1997-04-09 |
Family
ID=25441223
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
EP93305856A Expired - Lifetime EP0580452B1 (en) | 1992-07-24 | 1993-07-23 | Field effect trench transistor having lightly doped epitaxial region on the surface portion thereof |
Country Status (5)
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US (2) | US5910669A (en) |
EP (1) | EP0580452B1 (en) |
JP (1) | JP3387563B2 (en) |
KR (1) | KR100305978B1 (en) |
DE (2) | DE580452T1 (en) |
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Also Published As
Publication number | Publication date |
---|---|
KR940003092A (en) | 1994-02-19 |
JP3387563B2 (en) | 2003-03-17 |
US5532179A (en) | 1996-07-02 |
KR100305978B1 (en) | 2001-12-15 |
DE69309565T2 (en) | 1997-07-24 |
DE69309565D1 (en) | 1997-05-15 |
US5910669A (en) | 1999-06-08 |
DE580452T1 (en) | 1994-10-06 |
JPH06224437A (en) | 1994-08-12 |
EP0580452B1 (en) | 1997-04-09 |
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